The present invention relates to a semiconductor device using a silicon carbide layer, and more particularly to a silicon carbide semiconductor device that is capable of operating with a large current and has a high voltage resistance, and a method for manufacturing the same.
Silicon carbide (SiC) is a semiconductor having a higher hardness and a wider band gap than silicon (Si), and is used in power devices, environment resistant devices, high-temperature devices, high-frequency devices, etc.
A commonly-used power device is a switching device using Si. In a case where a switching device is used as a rectifier device, the device needs to have some voltage resistance. Therefore, a pn diode is used. However, a pn diode has a large switching loss. In view of this, a Schottky diode having a smaller switching loss is desirable. However, due to the physical limitation of Si, when a Schottky diode is formed by using Si, a desirable voltage resistance cannot be obtained. Thus, for realizing a switching device having a high voltage resistance and a small switching loss, SiC having a wide band gap has been attracting public attention.
FIG. 8 is a cross-sectional view illustrating a typical Schottky diode using SiC, which is a commonly-used switching device, of a first conventional example. As illustrated in FIG. 8, a Schottky diode 80 of the first conventional example includes a semiconductor substrate 81 made of n-type 4H—SiC, an n-type 4H—SiC layer 82 epitaxially grown on the upper surface of the semiconductor substrate 81, an ion implantation layer 83 into which boron, aluminum, or the like, is implanted, a Schottky electrode 84 made of nickel, titanium, or the like, provided on the upper surface side of the substrate and forming a Schottky barrier with the 4H—SiC layer 82, an ohmic electrode 85 made of nickel and provided on the reverse surface side of the semiconductor substrate 81, and an insulative layer 86 surrounding the Schottky electrode 84.
Herein, the ion implantation layer 83 is necessary for forming a guard ring structure for reducing the localization of an electric field, and is in contact with a portion of the Schottky electrode 84 along an interface 87. The ion implantation layer 83 functions to reduce the localization of an electric field occurring when a high voltage is applied between the Schottky electrode 84 and the ohmic electrode 85 so that the Schottky electrode 84 is on the negative side and the ohmic electrode 85 is on the positive side.
FIG. 9 is a cross-sectional view illustrating a typical Schottky diode using SiC, which is a commonly-used switching device, of a second conventional example. As illustrated in FIG. 9, a Schottky diode 90 of the second conventional example includes the semiconductor substrate 81 made of n-type 4H—SiC, the n-type 4H—SiC layer 82 epitaxially grown on the upper surface of the semiconductor substrate 81, the ion implantation layer 83 into which boron, aluminum, or the like, is implanted, a Schottky electrode 91 made of nickel, titanium, or the like, provided on the upper surface side of the substrate and forming a Schottky barrier with the 4H—SiC layer 82, the ohmic electrode 85 made of nickel and provided on the reverse surface side of the semiconductor substrate 81, and the insulative layer 86 surrounding the Schottky electrode 91. In contrast to the Schottky diode 80 of the first conventional example, the Schottky electrode 91 of the Schottky diode 90 of the second conventional example extends over a portion of the upper surface of the insulative layer 86.
The ion implantation layer 83 is necessary for forming a guard ring structure for reducing the localization of an electric field, and is in contact with a portion of the Schottky electrode 91 along an interface 92. The ion implantation layer 83 functions to reduce the localization of an electric field occurring when a high voltage is applied between the Schottky electrode 91 and the ohmic electrode 85 so that the Schottky electrode 91 is on the negative side and the ohmic electrode 85 is on the positive side.
Herein, in order for the ion implantation layer 83 to function as a guard ring in the Schottky diode 80 or 90 illustrated in FIG. 8 or FIG. 9, it is necessary to activate the implanted impurity such as boron through a high-temperature heat treatment. Specifically, in the process of manufacturing the Schottky diode 80 or 90, it is necessary to perform a heat treatment at a temperature higher than 1500° C. on the ion implantation layer 83 before forming the Schottky electrode 84 or 91.
Note that as disclosed in an article (Ito, et al., IEEE Electron Device Letters, Vol. 17, No. 3 (1996) pp139-141), for example, there are some reported cases in which the ion implantation layer 83 is subjected to a heat treatment at a relatively low temperature (1050° C.).
Moreover, in order to stabilize the characteristics of the Schottky electrode 84 or 91, a heat treatment at a temperature of about 400° C., for example, is performed before or after patterning a metal film forming the Schottky electrode 84 or 91.
However, it has been found that altered layers 88, 93 and 94 are formed in a region where the Schottky electrode 84 and the insulative layer 86 contact each other and in a region where the Schottky electrode 91 and the insulative layer 86 contact each other, as illustrated in FIG. 8 and FIG. 9. The altered layers 88, 93 and 94 are formed by the reaction between a metal of the Schottky electrode 84 or 91 and an insulative material of the insulative layer 86 occurring in a contact portion where the Schottky electrode 84 or 91 contacts the insulative layer 86 when performing a heat treatment on the Schottky electrode 84 or 91. If a significant electric field localization occurs in an area where the altered layer 88 or 93 contacts the insulative layer 86, a leak current may be induced, and the Schottky diode 80 or 90 may possibly be destroyed in some cases.
Moreover, if a heat treatment at a high temperature of 1500° C. or more is performed for activating the ion implantation layer 83, during the process of manufacturing the conventional Schottky diode 80 or 90, the surface of the 4H—SiC layer 82 may be roughened due to the influence of the high-temperature heat treatment, which may cause a leak current in the Schottky diode 80 or 90, and the various manufacturing process conditions may possibly change due to the loss of flatness of the surface. Moreover, if an impurity in the furnace attaches to the surface of the 4H—SiC layer 82 during the high-temperature heat treatment, a leak current may occur through the Schottky barrier due to the presence of an impurity at the interface between the 4H—SiC layer 82 and the Schottky electrode 84 or 91.
In order to prevent the roughening or contamination of the surface of the 4H—SiC layer 82, it is preferred to perform a heat treatment with a protection film such as an oxide film formed on the 4H—SiC layer 82. However, it is difficult to form a protection film that can withstand a heat treatment at a temperature as high as about 1500° C.
Furthermore, for performing a high-temperature heat treatment, a general-purpose apparatus cannot be used, but it is necessary to use a special-purpose high frequency induction heating apparatus such as an apparatus for growing a silicon carbide crystal. However, the use of such a high-temperature heating furnace requires some extra time for cooling, thereby lowering the throughput, and is thus disadvantageous in terms of the cost in mass production. Therefore, it is preferred to perform a heat treatment on the ion implantation layer 83 at a lower temperature. However, since the upper surface layer of the epitaxially grown layer 82 is damaged through the ion implantation process, the activation of the ion implantation layer 83 and the recovery of the crystal structure thereof may be insufficient if the heat treatment of the ion implantation layer is performed at a relatively low temperature.
A practical Schottky diode requires a forward current on the order of 1 A or more, and therefore requires a large electrode area. As a result, the area of the interface between the altered layer 88 or 93 and the ion implantation layer 83 is necessarily large. Therefore, the probability of a device breakdown due to the influence of the altered layer in the Schottky diode is not low.
Note that in some cases, the insulative layer 86 is absent in the structure illustrated in FIG. 8. However, in a case where the insulative layer 86 is absent, the majority of the ion implantation layer 83 is exposed to the outside, thereby leading to problems occurring during the assembly of the semiconductor device. For example, when a wire made of gold, aluminum, or the like, is connected to the Schottky electrode 84, the wire and the ion implantation layer 83 may contact each other, and the wire metal or the electrode metal may scatter, though in a slight amount, from the contact portion where the wire and the Schottky electrode 84 contact each other, if the ion implantation layer 83 is exposed. Therefore, a leak current or a device breakdown may occur unexpectedly.